In the various methods for making polyamines and mixtures thereof, often color bodies are formed which give the polyamine product an undesirable hue. Several processes have been proposed or used commercially for the removal of color bodies from mixed polyamine products. These processes encompass both physical and chemical methods.
For example, U.S. Pat. No. 4,766,247 relates to a process for the reduction of the color of polyamines by reacting at elevated temperature and pressure the colored polyamines, e.g. triethylenetetramine (TETA) or tetraethylenepentamine (TEPA), in the presence of a hydrogenation catalyst, e.g. Raney nickel, palladium or ruthenium on carbon, and a hydrogen atmosphere for a period of time sufficient to effectuate the desired reduction in color. In the '247 process, the polyamines can either be distilled into a narrow product composition and then hydrogenated or a crude polyamine product can be hydrogenated and then distilled to produce the desired product composition.
A process for decolorizing polyethylene polyamines, such as TETA, and higher homologues, such as TEPA, by treatment with active carbon at elevated temperatures followed by distillation is described in U.S. Pat. No. 3,723,529.
U.S. Pat. No. 3,595,921 describes that the color content of the polyethylene polyamine products prepared by the reaction of ethylene dichloride with ammonia is reduced by refluxing the product in the presence of at least about 0.5 wt. % potassium hydroxide for at least one hour at a temperature of about 110.degree. to about 190.degree. C. at the reduced pressure corresponding to the refluxing temperature. Polyethylene polyamine product having a substantially reduced color content is recovered from the refluxing mixture.
A process for decolorizing discolored polyethylene polyamines by distilling the polyamines in the presence of polyethylene polyamine hydrochloride is described in British Patent 1,351,050. Similarly, U.S. Pat. No. 4,570,019 describes a process which may be run continuously for producing polyethylene polyamines having improved color characteristics which comprises treating discolored polyethylene polyamines with polyethylene polyamine hydrochloride in the presence of water at elevated temperatures for the time necessary to reduce the color of the polyethylene polyamines. The decolorized polyethylene polyamines are then flash evaporated from the treating mixture.
A process for decolorizing discolored polyethylene polyamine by contacting the discolored polyamine with a chlorinated hydrocarbon, e.g. an alkyl chloride or an alkylene chloride, and then distilling the resultant mixture to separate the decolored polyamine is mentioned in European Patent 0 150 075 B1.
Finally, U.S. Pat. No. 4,731,165 teaches a process of decoloring crude TETA through contact with a sulfonic acid ion exchange resin. Subsequent distillation at elevated temperature obtains decolored TETA.
Some catalytic hydrogenation decoloration techniques use catalysts with short lifetimes. Others tend to be unduly complicated. Thus, there remains a need for new methods to decolorize polyamines without some of the disadvantages of the prior methods.